FIG. 14 is a circuit diagram illustrating the structure of a conventional multiphase current supplying circuit. A power supply system 1 includes an AC power supply 13, and supplies an AC voltage Vin to a diode bridge 2. An inductance parasitic to the power supply system 1 is indicated as an inductor 12 connected to the AC power supply 13 in series.
The diode bridge 2 performs full-wave rectification on the AC voltage Vin. An intervening circuit 3 is interposed between the diode bridge 2 and an inverter 4, and output from the diode bridge 2 is supplied to the intervening circuit 3. The intervening circuit 3 includes a capacitor 31 whose both ends are supplied with output from the diode bridge 2. The capacitance C of the capacitor is small, and selectively set at 20 μF, for example. The capacitor 31 can be miniaturized by reducing its capacitance value C.
A rectified voltage Vdc obtained at the both ends of the capacitor 31 is input to the inverter 4. In the inverter 4, switching of transistors serving as its switching elements of the inverter 4 is carried out based on switching signals Tu, Tv, Tw obtained from a control circuit 6. As a result, three phase currents iu, iv, iw are supplied to a motor 5.
The control circuit 6 is supplied with a phase θ1 of the AC voltage Vin, the rectified voltage Vdc, the currents iu, iv, iw, and a rotation position angle θm of a rotor of the motor 5. These respective quantities can be detected by well-known techniques. The control circuit 6 generates the switching signals Tu, Tv, Tw based on these quantities.
A technique is known with a significantly small capacitance value C of the capacitor 31 and appropriately controlled switching signals Tu, Tv, Tw based on the aforementioned respective quantities, to thereby carry out AC-AC conversion. Such switching control is herein referred to as capacitorless inverter control. The capacitorless inverter control allows for miniaturization of the overall circuit including a capacitor and an inverter and attains cost reduction as compared to an ordinary circuit that includes a smoothing circuit 301 or 302 (as shown in FIGS. 15 and 16, respectively) instead of the intervening circuit 3. While the smoothing circuit 301 adopts a smoothing large-capacitance capacitor CC and a power factor correction reactor LL, the capacitorless inverter control is capable of suppressing a power factor reduction on the power supply side without having to use such power factor correction reactor LL. And while the smoothing circuit 302 further includes a diode DD and a transistor QQ serving as a switching element to form a chopper circuit, the capacitorless inverter control is capable of suppressing power supply harmonics without having to use a chopper circuit.
The capacitorless inverter control is disclosed in a non-patent document 1, for example. In the non-patent document 1, an inverter is applied with a rectified voltage that pulsates widely with a frequency almost twice as much as that of a single-phase AC power supply. Yet appropriately controlled switching in the inverter allows three phase AC currents to be output. The non-patent document 1 indicates that, with respect to single-phase capacitorless inverter control, a power factor has a favorable value of 97% or more when a maximum value of both-end voltage of a capacitor is twice or more a minimum value thereof.
A patent document 1 is also cited as relevant to the present invention.
Patent document 1: Japanese Patent Application Laid-Open No. 2004-289985
Non-patent document 1: Isao Takahashi, “Inverter controlling method for a PM motor having a diode rectifying circuit with a high input power factor”, The Institute of Electrical Engineers of Japan, National Conference in 2000, 4-149 (March 2000), p. 1591